After bug #291759 was fixed - a hidden option was added to gwenview config file to display "lock zoom" button.
This button forces gwenview to remember zoom state while user going through images. But it only works in default mode. In fullscreen mode gwenview always resizes images to fit screen, and does not honor this "lock zoom" button.

Reproducible: Always

Steps to Reproduce:
1. run following command to enable "lock zoom" button:
 kwriteconfig --file gwenviewrc --group ImageView --key ShowLockZoomButton --type bool true 
2. open image in gwenview
3. zoom in
4. click "lock zoom" button
5. click "Full Screen" button
6. switch to the next image (hit "space" or click "next" button on panel)
 -- now you can see that new image get resized to fit the screen.


Expected Results:  
"lock zoom" button is expected to work in full-screen mode just like in normal mode.

As a sidenote - enabling the 'lock zoom' button is hard for normal user, because it requires to know what option to add to what config file. While the "lock zoom" feature itself is wanted by quite
a lot of users. 
There is an argument that extra button is cluttering up the "zoom area". But i think the button may be omitted at all and replaced with an option in config dialog or menu.